(1) Field of the Invention
This invention relates to electronic equipment for processing signals, particularly analog signals, such as those in the audio and video range, into corresponding digital signals suitable for recording on a record medium such as magnetic recording tape, and particularly relates to systems for correction of errors when such digitized signals are played back and are optionally reconverted into an output analog signal.
(2) Description of the Prior Art
The preparation of commercial recordings, both audio and video generally begins at the point where a number of tracks, such as 4, 8 or 16 tracks of analog audio signals are recorded onto a master tape, the master tape is then mixed with other masters to dub in other signals and the mixed masters are further mixed into sub-master tapes having a single monaural track, two stereo tracks or four quadraphonic tracks. The sub-master tapes are then used as the basis for manufacture of both record discs and prerecorded tapes. Any defects or deficiencies inherent in the analog signals are thus embodied in the subsequent generation master and sub-master recordings and become a part of all manufactured copies. In order to greatly reduce, if not completely eliminate many such deficiencies, professional recorders are now widely considering the adaptation of digital recorders such as are generally used in instrumentation and computer data processing fields for audio and video uses. In such recorders, as for example that depicted in U.S. Pat. No. 3,786,201 (Myers et al), the analog signals are periodically sampled and a digital word is generated corresponding to each sample. Since the signal to tape interface affects only the fine structure digit carrying signal and not the numerical content per se, the integrity of the digitized audio signal is maintained and no degradation in the quality of the recorded sound occurs, even with repeated re-recording, mixing and the like. Typical decreases in the amplitude or pulse rise time, etc. of the digital pulses may be recorded by conventional signal processing techniques.
However, despite the inherent desirability of such digital audio and video recorders, general acceptance in the professional recording industry has not yet been achieved. It is believed that at least part of the reason for the lack of acceptance has been the propensity of errors in the digitized signal as may result from defects in the record medium such as the familiar drop-out problem in magnetic recording tape. Rather than merely causing a momentary loss of signal as in conventional recorders, the loss of a digital bit may, if it occurs at a most inopportune time, cause the signal to lose sync entirely such that all subsequent portions of the digital signal are meaningless. To avoid such total loss, it is conventional to group the digitized data words formed from a number of bits into blocks or frames, each of which is indexed by a sync word. Such systems still do not prevent the loss of data within a given frame, which loss will still result in an undesirable shift in the output level or other disturbing electrical noises as well as the actual loss in the intended signals.
In order to prevent the loss of computer or other data processing information, systems have been developed for detecting the presence of errors in a playback signal and for correcting errors so detected. Typically, such data processing recorders enable error correction by providing redundant information which may then be recovered and played back in the event an error in a primary track is discerned. Most simply, such systems provide two (or more) totally redundant data tracks and record the same information on each of the tracks. Particularly, the data on the two tracks may be desirably spatially staggered along the length of the tape such that a single defect spanning both tracks will not cause the loss of the same portion of the signal. While such fully redundant systems are technically feasible, they obviously require twice as much record medium as would otherwise be needed. More sophisticated recorders have also been devised in which error correction codes are generated and recorded along with the digital data, such that when an error is detected, the correction codes are decoded to regenerate a corrected data portion corresponding to the erroneous data. Such schemes also generally utilize multiple tracks, in which one or more tracks may be exclusively devoted to storage of the error correcting code (ECC). See Patel, U.S. Pat. No. 3,745,528 reissued as U.S. Pat. No. Re 28,923. In that patent, the error detection operation provides error pointers (pointing to an erroneous block of data) which are generated by determining the quality of the playback signal, i.e., overall waveforms, etc.
However, not all data or recording systems are amenable to multitrack recording. Particularly, to facilitate compatibility with previously accepted recorder systems for use in audio and video recording, it is desirable to provide a single track digitized recorder in which an error correction operation is also employed. In U.S. Pat. No. 3,913,068 (Patel), a single track recorder is disclosed which utilizes a data format in which error check codes are included at the end of a block of data and in which external indicators are detected to initiate the need for error correction.
As an improvement over the systems discussed above, the above acknowledged related application of Brookhart, Ser. No. 021,177, is directed to a circuit for processing signals to be digitized and recorded on a single track of a suitable record medium. The circuit of that application includes an error correction feature which enables the reconstruction of corrected data in frame in place of erroneous data without the need for externally generated error pointers. The circuit is particularly adapted for use in a digital recorder in which both a record and a playback section are provided. The circuit includes means, such as an analog to digital converter, for converting an input analog audio signal into a corresponding digitized signal and an encoding means for partitioning the digitized signal into a serialized signal containing a succession of frames suitable for recording on the record medium. Each of the frames comprises a preselected number of data words, a preselected number of parity words, an error check word corresponding to the frame and a sync word defining the frame location. The encoding means includes means for generating the parity words of each frame by an exclusive -OR combination of data words of at least two other frames which are preselected according to the expression EQU P.sub.K.sup.N =D.sub.K+j.sup.N+n +D.sub.K+k.sup.N+m,
where
P.sub.K.sup.N is a given parity word located at segment K of frame N, PA1 D.sub.k+j.sup.N+n is a given data word located at segment K+j of frame N+n, and PA1 D.sub.K+k.sup.N+m is a given data word located at segment K+k of frame N+m.
In these expressions, K, j, k, m and n are all integers, and m and n are not equal to each other or to sero. The thus encoded parity words enable each frame that is incorrectly played back to be detected. The circuit further enables correct data words of the incorrect frame to be reconstructed from the data words of at least one of the other preselected frames in combination with the parity words originally generated from the data words of the incorrect frame and the corrected data words inserted into a serialized playback signal in place of the data words of the incorrectly played back frame.
Preferably, the preselected frames are in turn selected to comprise data words spatially located different predetermined intervals of time from the data words of the given frame such that the intervals of time are sufficiently long to provide a corresponding sufficient separation of the given frame from the preselected frames to minimize the probability that a single defect in a said record medium on which the digitized signal is to be recorded will cause the loss of signal corresponding to both the given frame and the preselected frames.
Analogously, the circuit desirably also includes a playback section including means for providing a digital playback signal corresponding to recorded data on a record medium, means for processing the digital playback signal to determine the presence of an erroneous frame, means for reconstructing a corrected data in a frame and inserting the corrected data in place of the erroneous data and means for converting the processed and corrected digital playback signal into a corresponding analog output signal. The means for locating erroneous data includes means for regenerating an error check code word corresponding to a received frame and means for comparing the regenerated error check word with a received error check code word of that frame to provide a frame error signal indicative of a said erroneous frame in the absence of a match between the two. The frame reconstruction means includes means for temporarily storing playback signals corresponding to each frame until signals corresponding to the preselected frames containing the parity and data words required for reconstruction of data words of said erroneous frame are received. When a frame error signal is received, corrected data words are reconstructed from the received parity and data words within said preselected frames and the reconstructed corrected data words are inserted in the proper spatial location within the processed digital playback signal.
The single track recorder with the error correction feature as described in the referenced application is particularly desirable in that it enables the expense of the record and playback heads to be minimized, and the record medium to be correspondingly narrower, thus facilitating handling the medium. In a preferred embodiment, an audio recorder having a one-inch tape moving at 45 ips has been found suitable for recording 32 parallel tracks in which the data in each track is protected by the error correction feature. Such a system is particularly useful in professional audio recorders where multiple track recording and mixing are desirable features. However, it has now been recognized that the configuration in which the data words from which the parity words are generated are all located ahead in time and spatially ahead on the tape of the parity words, results in the inability to reconstruct data words under certain conditions, and will result in the reconstruction of erroneous data words in the event a tape splice or overrecording is made.